This system, developed by Prof. Elihu Thompson, is much used to weld together wires, rods, small forgings and other parts which are made separately for quick production. It is used in welding links of chains, rings, wire fence-meshes, wheel tires, valves and their stems, lengths of steel steam pipe and their flanges, branch outlets, steam drums, etc. Different kinds of metal may be welded in one piece provided their melting points do not differ much.

Fig. 261. - Electric Welding Clamps.

The essential features of the welding apparatus of this system are shown in Fig. 261. The pieces R and 8 to be joined are held firmly together in the desired relative position by heavy copper clamps C and D, one of which may be moved toward the other. Current travels along the circuit through C, R, S and D, and when the two ends R and S are softened by heat, they are mashed together by the pressure of the clamps, forming an expanded burr which is later cut off. In some machines, however, a swage surrounds the joint at the time of welding, avoiding the burr and pressing the metal together very firmly in a lateral direction.

Alternating current is required in this system of welding. It heats both sides of the joint more evenly.